The present invention relates to a wireless communications system, and more particularly to a complex filter and a receiver circuit having the same.
Typically, in a wireless communications receiver system of a superheterodyne architecture, such as a low-IF architecture, a complex filter circuit is used for removing an image signal. FIG. 14 shows a configuration of a conventional low-IF type receiver circuit. An LNA (Low Noise Amplifier) 10 amplifies a quadrature-modulated signal received via an antenna (not shown), or the like. A quadrature demodulator 20 performs quadrature modulation while downconverting the received signal, which has been amplified by the LNA 10, to a medium frequency, thus producing an I signal and a Q signal having phases shifted from each other by 90 degrees. VGAs (Variable Gain Amplifiers) 30a and 30b amplify the I signal and the Q signal, respectively. A complex filter circuit 40A receives the I signal and the Q signal, which have been amplified by the VGAs 30a and 30b, respectively, and performs an image rejection operation on these signals. A signal processing section 50 having an A/D conversion function digitally converts the signals, which have passed through the complex filter circuit 40A, and performs various operations based on the digitally converted data.
An input to the complex filter circuit 40A, i.e., an output from the quadrature demodulator 20, in principle contains not only an intended signal but also contains, as a noise component, an image signal being a frequency component of the opposite sign to that of the intended signal. The complex filter circuit 40A passes an intended signal therethrough, thus functioning to reduce the image signal. Therefore, ideal transmission characteristics of a complex filter circuit are typically such that the gain is high for the frequency band of an intended signal and low for that of an image signal. The gain difference therebetween is called an “image rejection ratio”. A complex filter circuit is required to maintain a high image rejection ratio.
In the future, further reductions in the power consumption and size will be required for wireless communications systems. In order to reduce the power consumption and the circuit area of a complex filter circuit, it is effective to form the complex filter circuit as a Gm-C filter including a transconductor and a capacitor while setting the transconductance (gm) and the capacitance value (C) both to low values. However, setting these element values to small values increases an IQ mismatch, which is a mismatch between an element in the I signal processing portion of the complex filter circuit and the corresponding element in the Q signal processing portion thereof, thus degrading the image rejection ratio of the complex filter circuit.
FIG. 15 is a graph showing the transmission characteristics of complex filter circuits. As compared with ideal characteristics, the transmission characteristics of a complex filter circuit with an IQ mismatch significantly vary just over the frequency band of the image signal. Since the influence of the IQ mismatch is significant over the image frequency band, even a very slight error in an element may lead to a significant degradation of the image rejection ratio. Therefore, when one attempts to reduce the power consumption and the circuit area of a complex filter circuit, it is necessary to reduce the IQ mismatch in one way or another.
Conventional methods for reducing the IQ mismatch are based on the phase error in a local oscillator signal input to a quadrature demodulator. Specifically, the conventional methods aim at reducing the phase error in the local oscillator signal input to the quadrature demodulator because a phase error of the local oscillator signal leads to a phase error of the I and Q signals. With these conventional methods, however, it is essentially difficult to compensate for the gain error in the complex filter circuit.
Moreover, it is expected that pursuing further reductions in the power consumption and the circuit area of a complex filter circuit will result in the characteristics degradation due to an element error being more serious than that due to the phase error of the local oscillator signal. Therefore, it is necessary to establish a method for reducing the IQ mismatch based on the element error in the complex filter circuit.